Mixing multiple colored light emitting diodes (LEDs) to obtain a mixed color is a common way to generate white or colored light. The generated light is determined by the type of LEDs used, as well as by the mixing ratios. However, the optical characteristics of the LEDs change when the LEDs rise in temperature during operation: the flux output decreases and the peak wavelength shifts.
To overcome this problem, various feedback systems have been proposed in order to compensate for these changes in optical characteristics of the LEDs during use. These feedback systems provide an improvement in the color stability of the LED luminary. Examples of such feedback systems are disclosed in for example the documents WO03/037042 and WO02/47438. WO03/037042 discloses a LED luminary control system, which comprises a feedback unit generating feedback values representative of the actual mixed color light produced by the LED luminary. The feedback values are obtained from measurements by means of photodiodes. The system further comprises a controller for adjusting the LEDs in accordance with a difference between the obtained feedback values and reference or set point values representing a desired mixed color light. In this way, changes in LED characteristics can be compensated so that the LED luminary generates a desired mixed color light.
However, a problem with the above feedback system, as well as with other known feedback systems, is that in a realistic embodiment the photodiodes or other optical sensors detecting the actual output of LEDs will be integrated in the LED luminary. Consequently, not only the LEDs rise in temperature during operation but also the optical sensors. When the temperature of the optical sensors raises, the spectral sensitivity of the sensors changes due to a change in the sensor's quantum efficiency. This means that the measurements from the sensors are affected, which will lead to significant color change of the LED luminary. Already a temperature rise of about 60° C. can result in a clearly visible color change of the output of the LED luminary.